Direct contact cooling of processed food products has been widely used in the food processing industry since the 1960s. The direct contact cooling units, or “plate freezers” as identified within the industry, are traditionally constructed of individual planks which are coupled together through welding or other traditional means. Each of these planks contain internal passageways, through which a volatile refrigerant is introduced. The evaporation of the volatile refrigerant absorbs ambient heat and cools the surface of the planks, which in turn, cools the product which is resting on the surface of the planks. Traditionally, multiple planks forming the respective plates have been arranged in either vertical or horizontal orientations for large scale freezing operations.
Although somewhat effective in cooling various food products for example, the traditional direct contact freezers have fundamental performance, as well as potentially health hazardous, deficiencies. When cooling performance deficiencies manifest themselves in these prior art designs, the traditional industry response, heretofore, has been in the form of utilizing larger volumes of volatile refrigerant, with corresponding low coefficients of freezing. The need for large volumes of volatile refrigerant, as discussed above, has been viewed as a potential health hazard in that traditionally designed direct contact plate freezers have proven occasionally to leak refrigerant during repeated freezing or processing cycles.
Traditional direct contact plate freezers supply the volatile refrigerant to the associated freezing planks through flexible hoses or conduits. These flexible hoses are commonly connected to an intake manifold. Further, individual flexible hoses or conduits are typically coupled to a common suction manifold which is used for refrigerant removal. The respective hoses are traditionally connected to the aforementioned manifolds via a threaded stainless steel fitting. Inasmuch as the freezer planks are constructed of aluminum, there exists a difference in the linear coefficient of thermal expansion between the stainless steel connections and the respective aluminum planks. Because of this difference, the threaded connections are unavoidably the source of refrigerant leakage, and therefore poses an imminent threat to human health especially when refrigerants such as ammonia is employed.
Another issue facing manufacturers of frozen processed products is the inability to obtain a uniform distribution of the liquid refrigerant. Typically, the uniform distribution of the liquid refrigerant is accomplished by the use of fixed orifices which are mounted at each flexible hose connection. While this arrangement seems to work well, when subcooled liquid refrigerant is supplied to the common intake manifold, any flash gas entering the common intake manifold will rise to the top of the manifold and result in a restriction of the flow of the liquid refrigerant to the top or more elevationally oriented direct contact plates. It should be understood that flash gas is usually formed in the common intake manifold at the end of the freezing cycle if the flow rate of the refrigerant is reduced by throttling or the temperature of the refrigerant is permitted to approach its saturation temperature. The formation of flash gas, and the resulting non-uniform distribution of the liquid refrigerant causes unintended consequences in the freezing process. For example, as the top direct contact plates are “starved” of liquid refrigerant, the bottom direct contact plates have an abundance of liquid refrigerant. This situation results in an unequal freezing of individual items or products which are placed on the top direct contact plates (under-freezing), versus those placed on the bottom direct contact plates (over-freezing).
A similar problem associated with the non-uniform distribution of liquid refrigerant arises when the liquid refrigerant assumes a stratified or wavy flow pattern in the internal passageways of the respective direct contact plates. This stratified or wavy flow pattern is normally an artifact of the traditional construction of the internal passageways within the direct contact plates. The teachings of my U.S. Pat. No. 7,958,738 are incorporated by reference herein, and discuss this same problem.
Traditionally, to overcome the aforementioned problem of non-uniform distribution of the liquid refrigerant, and the formation of stratified or wavy flow patterns within the internal passageways of the direct contact plates, has been to significantly increase the flow rate of the liquid refrigerant. While increased flow rates of the liquid refrigerant will appear to address or mask, to some degree, the adverse effects of the non-uniform distribution of liquid refrigerant and the stratified or wavy flow patterns, it can result in overfeed ratios of the liquid refrigerant as high as 20:1. This overfeed of the liquid refrigerant can result in a significant waste of energy, the need for large volumes of liquid refrigerant, and the associated, potential health hazards posed by large volumes of a volatile liquid refrigerant, in the event that a liquid refrigerant leak occurs.
As most frozen food products are high value-added products, direct contact freezing performance, and throughput is critical to conducting a profitable operation. The problems associated with non-uniform distribution of liquid refrigerant, the need for large volumes of liquid refrigerant, and the associated human health hazards associated with a refrigerant leak, all potentially reduce the profitability of this industry.
It has long been known therefore, that it would be desirable to provide an improved direct contact plate freezing system which may be utilized in the frozen food industry, for example, and which avoids the inherent problems associated with the prior art practices and substantially reduces the potential for health problems for workers in close proximity to the traditional direct contact plate freezing systems discussed, above. Resolution of the above discussed deficiencies is the subject matter of the present invention, as will be described in greater detail hereinafter.